1. Field of the Invention
The present invention generally relates to an array antenna system. More particularly, the present invention relates to an apparatus and method for optimal beam-forming for transmitting and receiving high-speed data at high performance based on a regular spatial interpolation.
2. Description of the Related Art
Reception quality of radio signals is affected by many natural phenomena. One of the natural phenomena is temporal dispersion caused by signals reflected on obstacles in different positions in a propagation path before the signals arrive at a receiver. With the introduction of digital coding in a wireless system, a temporal dispersion signal can be successfully recovered using a Rake receiver or equalizer.
Another phenomenon called fast fading or Rayleigh fading is spatial dispersion caused by signals dispersed in a propagation path by an object located a short distance from a transmitter or a receiver. If signals received through different spaces, that is, spatial signals, are combined in an inappropriate phase region, the sum of the received signals is very low in intensity, approaching zero. This becomes a cause of fading dips where received signals substantially disappear, and the fading dip occurs as frequently as a length corresponding to a wavelength.
A known method of removing fading is to provide an antenna diversity system to a receiver. The antenna diversity system is provided with two or more spatially separated receive antennas. Signals received by the respective antennas have a low relation in fading, reducing the possibility that the two antennas will simultaneously generate the fading dips.
Another phenomenon is interference that is severe at the time of radio transmission. Interference is defined as an undesired component received through a desired signal channel. In a cellular radio system, interference is directly related to a requirement of communication capacity. Because resources of radio spectra are limited, a radio frequency band given to a cellular operator should be efficiently used.
Due to increasing use of cellular systems and their deployment over increasing numbers of geographic locations, research is being conducted on an array antenna geometry connected to a Beam-former (BF) as a new scheme for increasing traffic capacity by removing any influences of interference and fading. Each antenna forms a set of antenna beams. A signal transmitted from a transmitter is received in each of the antenna beams, and spatial signals experiencing different spatial channels are maintained by individual angular information. The angular information is determined according to a phase difference between different signals. Direction estimation of a signal source is achieved by demodulating a received signal. A direction of a signal source is indicated by a Direction of Arrival (DoA).
FIG. 1 illustrates an example of a Node B with an array antenna, which communicates with a plurality of User Equipment (UE) (or Mobile Stations (MSs)). Referring to FIG. 1, a Node B 10 has an array antenna 20 provided with four antenna elements. Five Users A, B, C, D and E are located in a service area of the Node B 10. A receiver 15 selects signals from desired users of the five users through beam-forming. Because the array antenna 20 of FIG. 1 has only the four antenna elements, the receiver 15 recovers signals from a maximum of four users, for example, signals from Users A, B, D and E as illustrated in FIG. 1 through beam-forming.
FIG. 2 illustrates, as an example, spatial characteristics of beam-forming for selecting a signal from User A. As illustrated in FIG. 2, a very high weight, or gain, is applied to a signal from User A, while a gain close to zero is applied in directions from the other users.
Estimation of a DoA is used to select an antenna beam for signal transmission in a desired direction or to steer an antenna beam in a direction where a desired signal is received. A beam-former estimates steering vectors and DoAs for simultaneously detected multiple spatial signals, and determines beam-forming weight vectors from a set of the steering vectors. The beam-forming weight vectors are used for recovering signals. Algorithms used for beam-forming are Multiple Signal Classification (MUSIC), Estimation of Signal Parameters via Rotational Invariance Techniques (ESPRIT), Weighted Subspace Fitting (WSF), and Method of Direction Estimation (MODE).
An adaptive beam-forming process depends on exact information about spatial channels. Therefore, adaptive beam-forming can generally only be accomplished after estimation of the spatial channels. This estimation should consider not only temporal dispersion of channels, but also DoAs of radio waves received at a receive antenna.
In an antenna diversity system using an array antenna, resolvable beams are associated with arrival directions of maximum incident waves. In order to achieve beam-forming, a receiver should acquire information about a DoA, and the information about the DoA can be obtained through DoA estimation. However, estimated DoAs are not regularly spaced apart from each other. Therefore, in a digital receiver, conventional beam-forming includes irregular spatial samplings. The ultimate goal of beam-forming is to separate an incident wave so as to fully use spatial diversity in order to suppress fading. However, its latent faculty is limited by the geometry of an array antenna having a finite spatial resolution.
Because a single-path channel is considered in a typical multipath and multiuser scenario, multi-path channels cannot be used in actual communication environments. Spatial selective channel estimation based on irregular spatial sampling proposed to solve this problem requires considerably complex implementation. Thus, a method based on regular spatial sampling has been proposed. The regular spatial sampling technique simplifies an Angle of Arrival (AoA) estimation and beam-forming process on the basis of the regular spatial sampling using a set of antenna elements uniformly distributed over the same circumference of a circle. This method estimates an AoA as a primary angle of an antenna element with the maximum received energy value.
FIG. 3 illustrates a structure of a receiver 300 of a conventional array antenna system, and FIG. 4 is a flowchart illustrating operations of an interference and noise estimator 340, a channel estimator 350, and a beam-former 360 in the receiver 300. Next, the respective components will be described in more detail.
Referring to FIG. 3, an antenna 310 is an array antenna with antenna elements of a predetermined combination structure, and receives a plurality of spatial signals that are incident thereupon through spaces. In an example of FIG. 3, incident plane waves are received in one direction at the antenna elements with different phases. Multipliers 320 multiply outputs of their associated antenna elements by antenna element-by-antenna-element weights set by the operation of the beam-former 360, respectively. A data detector 330 performs frequency down-conversion, demodulation, and channel selection on the weighted outputs of the antenna elements, thereby generating a digital data signal.
In step 410 of FIG. 4, the interference and noise estimator 340 estimates the interference power and the spectral noise density N0 of the thermal noise power using the data signal provided from the data detector 330. A covariance matrix indicative of the noise power is computed from a combined noise vector obtained by using the estimated interference power and the estimated spectral noise density. Because a received data signal is absent if a beam is first formed, the interference power is initialized to an arbitrary value and therefore the noise power is computed.
In step 420, the channel estimator 350 computes a phase matrix As, including a phase factor Φ associated with User k and Antenna Element ka using Nb predetermined DoA values, computes directional channel impulse response vectors ĥd using Equation (1), and computes combined channel impulse response vectors h by multiplying the directional channel impulse response vectors by the phase matrix.ĥd=(AsH(Ika{circumflex over (x)}GH)Rn−1(Ika{circumflex over (x)}GH)−1AsH(Ika{circumflex over (x)}GH)Rn−1e  Equation (1)
In Equation (1), the matrix G is a midamble known between a transmitter and a receiver and e is a combined received signal vector. Ika is a (ka*ka) identity matrix and Rn is a covariance matrix indicative of the total noise power between antenna elements.
In step 430, the channel estimator 350 evaluates channel estimates with antenna element-by-antenna-element energies with respect to the directional channel impulse response vectors, and ranks energies ∥ĥd(k,na)∥2 of the directional channel impulse responses estimated in directions na for all K users in order of magnitude.
In step 440, the channel estimator 350 selects one direction with the maximum impulse response energy for each user, maintains only the channel impulse response energy in the selected direction, sets energies of all other channel impulse responses to zero, and forms modified directional channel impulse response ĥd,mod. The modified directional channel impulse responses are used to compute the final combined channel impulse vectors ĥ along with the phase matrix As using the fixed DoA values.
In step 450, the beam-former 360 computes steering vectors for adaptive beam-forming in all directions on the antenna element-by-antenna-element basis using the computed combined channel impulse response vectors ĥ. In step 460, the beam-former 360 performs beam-forming using the combined channel impulse response vectors and the steering vectors in estimated DoA of incident waves.
The beam-forming method based on the above-described regular spatial sampling can simply implement a system because it is simpler than an adaptive beam-forming method. In terms of implementation, the beam-forming method based on the above-described regular spatial sampling can obtain a significant gain. However, it is difficult for this method to correctly estimate the location of a target UE because a given space is regularly divided and a direction/angle of a signal arrived in a region of the divided space is indicated by one DoA value. Accordingly, the performance of the beam-forming system is degraded. The performance can be improved by increasing the number of antenna elements to solve the problem. When this occurs, the complexity and cost of the system increase.